use feature 'dor' was an alias for use feature 'err', there wasn't a dor keyword.
There is a longish thread at http://www.nntp.perl.org/group/perl.perl5.porters/2007/09/msg128170.html which discusses the reason for removing the err keyword. It boils down to people associating err with errors and therefore too likely to clash with current practice and confuse those new to the operator.
The // operator remains and since it doesn't clash syntactically with the earlier use of // (one is an operator the other a term) there is no need for use feature 'err'.
Update: as pointed out by ysth below I'd missed the significance of http://www.nntp.perl.org/group/perl.perl5.porters/2007/09/msg128324.html where Rafael Garcia-Suarez writes "Larry decided to rename err to orelse in Perl 6.". Thanks to ysth for pointing it out.